And then there is glow that is not related
to making something hot, called luminescence. The kind of luminescence depends
on specific details of the quantum structure of the material, but falls into
some major categories:

Fluorescence

Luminescence in which visible light is emitted from
a substance under excitation by ultraviolet light or other forms of
electromagnetic
radiation. The light is given off only while
the stimulation continues. Fluorescence of certain rocks and other substances
has been observed for hundreds of years. The wavelength of the fluorescent
light is always greater than that of the exciting radiation.

Phosphorescence

Luminescence produced by certain substances after absorbing
radiant energy or other types of energy. Phosphorescence continues even
after the radiation causing it has ceased. Energy is absorbed by a phosphorescent
substance, causing some of the electrons of the crystal to be displaced.
These electrons become trapped in potential troughs from which they are
eventually freed by temperature-related energy fluctuations within the
crystal. As they fall back to their original energy levels, they release
their excess energy in the form of light.

Chemiluminescence

The name given to those cases in which chemical action produces light
without any great rise of temperature. Phosphorus exposed to moist air
in a dark room shines with a soft light due to slow oxidation. Decaying
wood and other vegetable substances often exhibit the same property.

Thermoluminescence

Some materials that do not emit light at ordinary temperatures
in a dark room begin to do so if they are heated to a temperature below
a
visible
red heat. In the case of chlorophane, a variety of fluor-spar, the heat
of your hand is sufficient.

Electroluminescence

Lluminescence due to electrical causes. Many gases are phosphorescent
for a short time after an electric discharge has been passed through them,
and some solid substances are strongly
phosphorescent when exposed to a beam of electrons.

Triboluminescence

Luminescence excited by friction, percussion, cleavage or other mechanical
means. Popularly, Winter Green Lifesavers are famous for this effect. The
ultraviolet light produced by cleaving sugar crystals is converted into
a flash of visible light by the organic dyes and flavoring of the wintergreen
mint.

Fluorescence - Cool Glow

Triboluminescence - WintOGreen Lifesaver

While all forms of luminescence are interesting to think about, our focus
is on phosphorescence this time.